Network operators and carriers are deploying packet-switched communications networks in place of circuit-switched networks. In packet-switched networks such as Internet Protocol (IP) networks, IP packets are routed according to routing state stored at each IP router in the network. Similarly, in Ethernet networks, Ethernet frames are forwarded according to forwarding state stored at each Ethernet switch in the network. The present invention applies to communications networks employing any Protocol Data Unit (PDU) based network and in this document, the terms “packet” and “packet-switched network”, “routing”, “frame” and “frame-based network”, “forwarding” and cognate terms are intended to cover any PDUs, communications networks using PDUs and the selective transmission of PDUs from network node to network node.
Multi-Link Trunking, which is described in IEEE 802.3-2005 (Section 43), provides a method by which two peer Client Systems (CSs) connected by two or more point-to-point full duplex links, can cooperate to treat the parallel links as a single “link”, referred to as a Link Aggregation Group (LAG). As may be seen in FIG. 1, this is accomplished by implementing an aggregator (AG) 4 at each peer CS 6. The Aggregator 4 is logically interposed between the Media Access Control (MAC) Client and the MAC layer (not shown) of the involved network node, and generally comprises a Distributor for distributing frames to individual ports for transmission; a Collector for collecting received frames from individual ports and forwarding them to the MAC Client; and a controller for managing the state of the link group. Thus, at a sending CS 6a, the distributor routes packets from the MAC Client to a selected one of the LAG links 8, in such a way as to achieve desired load balancing and resiliency. At the receiving CS 6b, the collector forwards packets received through the LAG links 8 to the MAC Client. With this arrangement, the existence of multiple links 8 within the LAG 2 is rendered transparent to each of the respective MAC clients of the peer CSs 6.
Multi-Link Trunking provides increased bandwidth and inherent resiliency between two directly connected peer nodes beyond that which can be achieved with a single link but in a form that preserves key properties that are associated with a single link. It would therefore be desirable to extend Multi-Link Trunking across a network. That is, it would be desirable to set up an “extended link aggregation group” between a pair of client systems, in which each “link” of the extended link aggregation group traverses an independent (and physically diverse) set of links and nodes between the two involved client systems. Preferably, the “extended link aggregation group” would be configured in such a way that each of the client systems would utilize conventional MAC Client and Aggregator functions as defined in IEEE 802.3-2005 (Section 43).
Split Multi Link Trunking (SMLT) is a technique developed by Nortel Networks Limited, and is described in co-assigned U.S. Pat. No. 7,269,132 and an internet draft entitled “Split Multi-Link Trunking (SMLT)” http://tools.ietf.org/html/draft-lapuh-network-smlt-07, which extends Multi-Link Trunking. As may be seen in FIG. 2, Split Multi Link Trunking (SMLT) replaces one of the peer CSs 6 with a pair of physical Ethernet Switches (ESs) 10. Because the link aggregation group (LAG) has been split across two physical switches, it may be referred to as a Split Link Aggregation Group (SLAG) 12. At the “split” end 14 of the SLAG 12, the aggregator and MAC Client functions (of the replaced peer CS 6b) are distributed between the two SMLT-enabled ESs 10, so that the single aggregator 4a and CS 6a at the other end of the SLAG 12 are not aware of the split. A special link 16 (which itself may be an aggregated group) is provided between the two SMLT-enabled ESs 10, to enable coordination between the respective parts of the distributed aggregator and MAC client functions. This special link 16 is referred to as an Inter-Switch Trunk (IST). Applicant's co-assigned U.S. Pat. No. 7,269,132 teaches methods by which conventional MAC client and Aggregator functions can be retained at the “joined” end of the SLAG group, by representing the peer ESs 10 at the split end 14 of the SLAG 12 as a single phantom node (not shown in FIG. 2).
As may be appreciated, SMLT enables a resilient connection between a client system (CS) and an Ethernet domain, by enabling traffic to or from the CS to be routed through the pair of SMLT enabled Ethernet switches. Thus it provides a “first step” in establishing an extended link aggregation group which spans an Ethernet domain, by enabling each CS to connect to the Ethernet domain via a respective pair of SMLT-enabled ESs. However, there is no simple way of extending the link aggregation group across the Ethernet domain between the peer SMLT-enabled ESs and, for example, a second set of peer SMLT-enabled ESs supporting a second CS.
As noted above, the conventional MAC client and Aggregator functions can be retained at the “joined” end of the SLAG group 12, by representing the peer ESs 10 at the split end of the SLAG 12 as a single “phantom” node. The use of the phantom node also allows Spanning Tree Protocol (STP) to be used in a conventional manner to set up a connection through the Ethernet domain from the peer ESs 10 of the SLAG 12 to a desired destination address (such as counterpart peer ES of another SLAG). In principle, this could enable respective paths to be set up through each of the peer ESs 10. However, in order to prevent looping, and consequent failure of the network, one of these paths must be logically disabled. As a result, while packets sent from a CS 6 can arrive at either of the peer ESs 10 of the SLAG 12, both packet flows will be combined into a single connection for transport across the Ethernet domain.
Techniques for extending a link aggregation group across a network which overcome at least some of the above-noted issues remain highly desirable.